1. When your Grand Offensive is victorious you can continue it for more
turns. When do you have to stop? Is that voluntarily?

Yes, you may end it voluntarily when you want either: at START of your
Game turn or at the END of the current turn (after both players have played).
The required end offensive rules still apply anyway. At last, you have to stop
at the end Nov- December turn.

2. Are orders of battle (rule 54.C) used in a secondary battle?

No! The battle orders are only used for the Main battle of the Grand
Offensive. See 52.C (Order of battle: none).

3. Obligation for an offensive. (53.D). Is Italy - when in the war - obliged
to conduct a Grand Offensive each year? If the answer is yes, does that include
the year in which Italy enters the war?

Yes, Italy has to launch a Grand Offensive each year, except the year
when she enters at war: as the Grand Offensive is planned at the New Year's
Start (during the Winter Interphase).

4. Turkey has no such obligation (as only secondary battles are allowed in
the Turkish theaters of war).

Yes, Turkey has no such obligation, as she only can launch secondary
battles in the Turkish theaters of war (Balkan, Armenia, Egypt... no Grand
Offensive allowed).

5. Putnik and Albert have an obstinate value of '0'. How does that work? Can
they enter a hex for battle, and then, when the enemy is revealed, cancel it
right away?

No, it was a very "minor" error printed in their counter.
Putnik and Albert worth value as "1" obstinate. They have to attack 1
round at least.

6. Can attackers' heavy arty from GHQ intervene in bombardment in secondary
battles?

Yes.

7. (Rule 52.C): Defender may make his heavy arty from GHQ intervene. I don't
understand why this is so, because defender's arty plays no role in bombardment
procedure in secondary battles.

Yes, you are right for the bombardment. The reason is different: then the
Defender may make his heavy arty from GHQ intervene (during the bombardment
time) and use it later during the battle.

8. (Rule 55.M) Swiss and Belgian units can cooperate inside and outside
their own countries. This means French or British corps can reinforce them. Can
French or British (GHQ) artillery also be used as reinforcement in a battle
involving a Swiss or Belgian stack?

Yes, French / British / (US or InterAllied) GHQ may send reinforcements
in a battle involving a Swiss or Belgian stack. It is unclear in the rules, but
yes.

Note: You may even add Holland (if invaded?). The Centrals act
as well in the very rare case if the Allies invade Belgium, Holland or
Switzerland.